CMOS imagers are increasingly being used as low cost imaging devices. A CMOS imager circuit includes a focal plane array of pixel cells. Each of the pixel cells includes a photosensitive element, such as a photodiode, photogate, or photoconductor overlying a doped region of a substrate for accumulating photo-generated charge in an underlying portion of the substrate. A readout circuit is connected to each pixel cell and often includes a floating diffusion region for receiving charge from the photosensitive element, and a source follower transistor, which has a gate electrically connected to the floating diffusion region. The imager may also include at least one transistor for transferring charge from the photosensitive element to the floating diffusion region, and a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transfer. A row select access transistor is also typically used to gate a pixel output signal produced by the source follower transistor. The pixel cell above is often called a CMOS Active Pixel Sensor (APS) cell, which is used to collect light energy and convert it into a readable electrical signal.
A schematic top view of a portion of a semiconductor wafer fragment containing one exemplary CMOS APS cell is shown in FIG. 1. This CMOS APS cell 10 is a four transistor cell. As it will be described below, the CMOS APS cell 10 shown includes a photodiode 13 formed within a substrate. This photodiode 13 is formed as a pinned photodiode shown in FIG. 2. Alternatively, the CMOS APS cell 10 may include a photogate, photoconductor or other photon to charge converting device, in lieu of a pinned photodiode 13, as the initial accumulating area for photo-generated charge. The photodiode 13 includes a p+ surface layer 5 and an underlying n− region 14.
The CMOS image sensor 10 of FIG. 1 has a transfer gate 7 for transferring photoelectric charges generated in the n− region 14 to a floating diffusion region (sensing node) 3. The floating diffusion region 3 is further connected to a gate 27 of a source follower transistor. The source follower transistor provides an output signal to a row select access transistor having gate 37 for selectively gating the output signal to terminal 33. A reset transistor having gate 17 resets the floating diffusion region 3 to a specified charge level before each charge transfer from the n− region 14 of the photodiode 13.
Referring to FIG. 2, pinned photodiode 13 is formed on a p+ substrate base 1, which is beneath a p− epitaxial layer 2 (or p-well surface layer). Typically, the substrate base is p-type and is beneath a p-type epitaxial layer. It is also possible, for example, to have a p-type substrate base beneath p-wells in an n-type epitaxial layer. The n− region 14 underlies the p+ region 5 at a depth in the epitaxial layer 2 above the substrate base 1. The n− region 14 and p+ region 5 of the photodiode 13 are spaced between an isolation region 9 and a charge transfer transistor gate 7. The pinned photodiode 13 has a p+/n−/p− structure.
The photodiode 13 has two p-type regions 5, 2 having a same potential so that the n− region 14 is fully depleted at a pinning voltage (Vpin). The pinned photodiode is termed “pinned” because the potential in the photodiode is pinned to a constant value, Vpin, when the photodiode is fully depleted. When the transfer gate 7 is operated, photo-generated charge is transferred from the charge accumulating region 14 to the floating diffusion region 3. A complete transfer of charge takes place when a voltage on the floating diffusion region 3 remains above Vpin while the pinned photodiode functions at a voltage below Vpin. An incomplete transfer of charge results in image lag.
The isolation region 9 is typically formed using a conventional shallow trench isolation (STI) process or by using a Local Oxidation of Silicon (LOCOS) process. The floating diffusion region 3 adjacent to the transfer gate 7 is commonly n-type. A translucent or transparent insulating layer (not shown) may also be formed over the CMOS APS cell 10.
Additionally, impurity doped source/drain regions 32 (FIG. 1), having n-type conductivity, are provided on either side of the transistor gates 17, 27, 37. Conventional processing methods are used to form, for example, contacts 32 (FIG. 1) in an insulating layer to provide an electrical connection to the source/drain regions 32, the floating diffusion region 3, and other wiring to connect to gates and form other connections in the CMOS APS cell 10.
Generally, in CMOS pixel cells, such as the CMOS APS cell 10 of FIGS. 1 and 2, incident light causes electrons to collect in n− region 14. An output signal produced by the source follower transistor having gate 27 is proportional to the number of electrons to be extracted from the n− region 14. The maximum output signal increases with increased electron capacitance or acceptability of the n− region 14 to acquire electrons. In this example, the p+/n− junction dominates the capacitance of the pinned photodiode 13. The characteristics of a p/n junction are well known.
At least approximately 30,000 electrons are needed to generate an adequate photosignal. Even as pixel size and, therefore, photodiode size, scale down, the number of electrons needed for an adequate photosignal remains the same. Decreasing the size of a conventional photodiode decreases its electron capacitance, leading to an inadequate photosignal. Accordingly, a pinned photodiode for use in a CMOS APS cell that permits decreased pixel size without decreasing electron capacity or increasing the pinning voltage is desired.